April 27, 2008

Gasoline to Battery Range Comparison

One tank of gasoline contains hundreds of millions of Joules of energy (MJ), about 45 MJ/kg according to Wikipedia. Let’s say your typical car is 3,000 lbs in weight, has a range of 350 miles on a tank of gas, and gets about 30 mpg highway.

By contrast a Nickel-Metal Hydride (NimH) battery is good for about 0.22 MJ/kg. For the sake of discussion, using today’s technology how much battery mass would it take to provide the equivalent?

At 30 mpg it would take 11.67 gallons of gas to go 350 miles. If each gallon is roughly 6.5 lbs, then we have roughly 76 lbs or 34.5 kg of gas. Based on 45 MJ/kg that’s 1,552 MJ of energy.

We know a 350 mile range would be too far for batteries. So how much would the market accept as an alternative? Let’s assume 2/3 of that which would be 210 miles. For the same vehicle then 210 miles would require 2/3 as much energy which would be equal to about 1,035 MJ. But keep in mind that a gas engine is about 25% efficient whereas an electric power train is closer to 75%.

That means if you’re using 1,035 MJ of gas at 25% efficiency, you would only need about 345 MJ of electricity (1,035 x 0.25 / 0.75).

Using battery tech with 0.22 MJ/kg we’d still need an astounding 1,568 kg (or 3,456 lbs) of batteries. No wonder the ranges being discussed for plug-in hybrids (PHEV) are more often in the 50 to 100 mile range, using lithium batteries.

However, all is not lost. If we can pare the weight of the vehicle down from 3,000 lbs to say 1,500 lbs we can probably save another 1/3 in the energy for the same range. If that was done battery mass would come down proportionally to 1,045 kg (2,305 lbs). Still not practical but that’s for a 200 mile range. If the range were cut to 50 miles then it looks like we could get away with less than 600 lbs of batteries (2,305 lbs x 50 miles/200 miles = 576 lbs) if the car is very light. This does not yet account for the weight of the batteries either.

It’s clear there are only 3 main ways to increase PHEV range without additional fuel:

While there’s a lot of work being done on improving battery technology, it should be noted that dramatically lighter vehicles will strongly contribute to the growth of PHEV vehicles. And in doing so we’ll probably see a lot of refreshing concepts in the very near future.